In the manufacture of a spark plug, a ceramic insulator body is positioned in a hollow metal shell and an annular end lip on the metal shell is crimped inwardly onto an adjacent annular shoulder on the insulator body to secure the insulator body and the metal shell together and also to move the insulator body in an axial direction to compress (plastically deform) an internal annular gasket positioned between the insulator body and the outer metal shell. The metal shell may include a reduced thickness section (referred to in the art as a "weld groove") disposed between the crimped lip and the internal seal. The weld groove typically is heated by suitable means, e.g., by electrical resistance heating, to an elevated temperature during or after the crimping operation. Upon cooling to ambient temperature after the crimping operation, the reduced thickness section of the metal shell contracts and imparts an increased sealing pressure on the deformed gasket by virtue of differences in the coefficient of thermal expansion between the assembled metal shell and the ceramic insulator body.
Crimping of the annular lip of the metal shell is typically effected by advancing a crimping die at a given die pressure or load axially along the shell and insulator body to plastically deform the lip while a stationary die supports the metal shell against movement at an outer annular seat on the shell (referred to in the art as the "engine seat"). In the past, a typical crimping die has included an annular working face which, when viewed along an axial cross-section, has a shape corresponding to a partial circle of a selected radius.
With the advent of high speed systems for manufacturing spark plugs wherein the "weld groove" is heated for a time on the order of three seconds prior to crimping of the annular lip of the metal shell and sealing of the shell and insulator body, problems with insufficient internal sealing and excessive deformation of the metal shell, especially at the "weld groove" and the "engine seat" thereof, have been encountered and have resulted in the production of unacceptable spark plugs.
There is a need to provide an improved crimping die as well as crimping process especially useful for the high speed production of spark plugs that overcomes these problems and results in a spark plug with acceptable internal sealing of the gasket between the insulator body and the outer metal shell and with deformation of the metal shell reduced within acceptable limits.